1. Field of the Invention
The subject invention relates to the preparation of heat-curable bismaleimide resin systems. More particularly, the subject invention relates to the preparation of bismaleimide resin systems in which the bismaleimide component(s) are slurry mixed at relatively low temperatures into the remaining resin system components. The greater homogeneity of the resulting resin systems allows the preparation of prepregs and adhesives with uniform properties, and unexpectedly increases the tack and drape of such products.
2. Background of the Related Art
Bismaleimide resins have recently become important resins of commerce for a variety of uses, for example, in printed circuit boards, as matrix resins for fiber reinforced composites, as structural adhesives, and as potting resins, to name but a few. However, bismaleimide resins, while possessing high strength at elevated temperatures, tend to be somewhat brittle. Thus it is common to modify the bismaleimides with additional comonomers to improve impact resistance. However, many of the comonomers are physically incompatible with the bismaleimide resins in the sense that following initial formulation at elevated temperatures, the bismaleimide component may crystallize during processing. The large crystals which then form result in a resin system which is heterogenous, difficult to process and which may be prone to microcracking when used to prepare carbon fiber reinforced composites. In such a resin system the bismaleimide concentration can vary widely. Furthermore, even when the bismaleimide components and other components are physically compatible, adhesives and prepregs prepared from these conventionally prepared systems often do not have the drape or tack desired of them.